The present invention relates to an image forming apparatus used for copying machines, printers, multi-function peripherals or the like having a copy function and a print function. Moreover, the present invention relates to an optical multi-beam scanning device used for such image forming apparatus. For example, the optical multi-beam scanning device and the image forming apparatus of the invention can be applied to the apparatus using color.
For example, in a conventional optical multi-beam scanning devices in which four scanning lines are formed, one polygon mirror is employed so that a portion of optical elements or all the optical elements in a post-deflection optical system are shared. In such scanning devices, when the four beams are scanned by one polygon mirror surface, for example, as disclosed in the U.S. Pat. No. 5,715,078 or the U.S. Pat. No. 5,838,479, each beam from each light source is adapted to be directed (namely, to be combined) to a same polygon mirror surface in order of arrangement in a sub-scanning direction. Between each light source from which each light beam is emitted and the polygon mirror surface, there is provided a pre-deflection optical system with respect to each beam, which performs predetermined functions such as adjustment of sectional shape of each beam, and each beam is directed to the same polygon mirror surface by means off optical elements and so on used for combination after it passes through the pre-deflection optical system.
However, each beam from each light source is combined to a common optical path in order of arrangement in the sub-scanning direction in a conventional method in which combining to four beams is performed. For that reason, it is required a longer distance in the sub-scanning direction between beams from each pre-deflection optical system. Otherwise, since a larger angle between respective beams is required, a sub-scanning direction incidence angle onto the polygon mirror surface must be made larger with respect to the beams of both ends in the sub-scanning direction.
In the former case, the width in the sub-scanning direction of the polygon mirror surface increases and this causes the increase in the polygon mirror size, whereby windage loss and wind noise are easily generated. In the latter case, sub-scanning direction displacement of an image surface caused by the polygon mirror surface irregularity (variation in distance from a center of rotation of-each surface) becomes greater to cause degradation of image quality. (it is proportional to an amount of mirror surface irregularity and proportional to the incidence angle in sub-scanning direction onto the polygon mirror surface.) Further, the longer distance between beams from each pre-deflection optical system or the larger sub-scanning direction incidence angle onto the polygon mirror surface of each beam makes sub-scanning direction thickness of the optical elements shared with the four beams in the post-deflection optical system greater.
That is to say, the above factors are the obstacles that hamper thinning and miniaturizing in the sub-scanning direction. In addition, the positions of optical elements for combining the plurality of beams must be disposed on such positions that are greatly distanced in the sub-scanning direction between beams. In other wards, greater distance from the polygon mirror is required to dispose the optical elements, and this is the obstacle for miniaturizing the optical multi-beam scanning device.